This means that if a category has no restrictions at all, then any products in it will always be allowed, whatever other categories they are in. If you instead wish empty categories to be ignored when handling products in multiple categories, then activate this option.

This means that if a category has no restrictions at all, then any products in it will always be allowed, whatever other categories they are in. If you instead wish empty categories to be ignored when handling products in multiple categories, then activate this option.